FIG. 1 shows a conventional plain old telephone service (POTS) device 11 that is connected to a 2-wire (tip and ring lines) copper telephone line 12, also known as subscriber loop. The POTS device 11 can be a telephone, analog modem, facsimile machine, etc. As shown, the POTS device 11 may include a ringer 14, for example, in the case of a telephone, that is connected to the tip and ring lines associated with the telephone connection 12. A telephone line connection switch 16, for example, a hook switch in the case of a telephone, serves to electrically connect and disconnect the POTS device 11, except for the ringer 14, if applicable, to and from the telephone connection 12, as appropriate. In the case of a telephone, when the telephone is “on-hook,” the switch 16 serves to isolate the telephone polarity guard 18 and transmitter/receiver 19 from the telephone line 12, and when the telephone is “off-hook,” the switch 16 serves to connect the telephone polarity guard 18 and transmitter/receiver 19 to the telephone line 12. Polarity guard 18 connected to the switch 16 essentially enables the tip and ring connections from the POTS device 11 to be connected to either the tip and ring connections, respectively, or the ring and tip connections, respectively, of the telephone line 12. The polarity guard 18 thus accommodates tip/ring polarity reversals to the POTS Device 11. Finally, a transmitter, receiver, or both (transceiver) 19 is connected to the polarity guard 18 for transmitting and/or receiving signals. The transmitter, receiver, or transceiver, denoted collectively by reference numeral 19, implements the modulation and other signal processing functions, as appropriate.
Recently, in the art of telephony, the 2-wire telephone line 12 has been utilized for communicating more than one simultaneous communications signals, such as one or more digital data signals in addition to the analog POTS voice signals. For example, a high speed digital subscriber line (xDSL, e.g., asymmetric DSL (ADSL), symmetric DSL (SDSL), rate adaptive digital subscriber line (RADSL), very high speed DSL (VDSL), ISDN-based DSL (IDSL), etc.) channel and a POTS channel can be established over a single physical 2-wire connection. The signals are typically separated in frequency. The POTS channel usually exhibits a frequency spectrum of about 0 KHz to about 4 KHz, whereas the xDSL channels exhibit a frequency spectrum of about 20 KHz to about 1 MHz.
A POTS splitter 21, also known as a telephone filter, a microfilter, or a CP (customer premises) filter, has traditionally been utilized to decouple the channels, or separate the POTS channel from the other channels, due to non-linearities inherent in most, if not all central office powered telephones. The CP POTS splitter is usually implemented in series with the POTS device 11 in order to reduce the xDSL power imparted on the POTS device electronics, which imposes interference upon the POTS and xDSL signal.
A POTS splitter 21 is typically a passive or active two or three port device. The POTS splitter 21 is installed at the customer premises (CP) between a telephone jack and the POTS device 11 itself and is also connected to a DSL device 23 at the CP. It includes a POTS filter (a two port device) situated between the telephone line 12 and the POTS device 11 that is designed to minimize high frequency transients produced by on-hook/off-hook transitions, so as to prevent tainting or slowing of the high speed data on the DSL channel. Also, it is usually configured to provide a high impedance to the telephone line 12 in the DSL frequency band in order to prevent DSL power from being shunted and reduced by POTS device 11 that is connected to the line 12. Also, the splitter 21 reduces incident DSL signal power to reduce DSL signal intermodulation distortion (IMD), which undesirably creates noise in the receiver of the POTS device 11 (which in the case of a telephone, can be heard over the handset). DSL signal IMD can also interfere with DSL modem operation.
The specification of POTS splitters 21 has been the subject of several industry standards bodies. For example, see American National Standards Institute, ANSI T1.413-1995, Sections 8 and 10, regarding ADSL/POTS splitters. Moreover, as an example of a possible implementation of a POTS splitter, see J. Cook, P. Sheppard, “ADSL and VADSL Splitter Design and Telephony Performance,” IEEE Journal on Selected Areas in Communications, December 1995.
Because of the foregoing requisite functionality, POTS splitters 21 are usually expensive devices and are oftentimes installed on a physical wall of the CP, such as on an outside wall of a building. Furthermore, generally, POTS splitters 21 require installation by a skilled telephone company worker, not the premise owner, resulting in undesirable installation time, labor, and expense.
Commonly assigned U.S. Pat. No. 5,848,150 to T. J. Bingel, entitled, “PASSIVE DISTRIBUTED FILTER SYSTEM AND METHOD,” describes a distributed POTS filter system employing a plurality of passive two port POTS filters. Each POTS filter (not shown in FIG. 1 for simplicity) is situated between the telephone line 12 and a respective POTS device 11 for enabling decoupling of a POTS channel from one or more xDSL channels that are communicated simultaneously with the POTS channel along the telephone connection. The POTS filter can also be situated within a POTS device 11, instead of on the outside.
Although meritorious and effective to an extent, the POTS splitters and filters add undesirable complexity and circuitry to the systems. Furthermore, the POTS splitters and POTS filters usually contain an inductor(s) in series with the telephone line and a capacitor(s) in parallel with the telephone. The presence of these reactive elements introduces concerns about excessive POTS attenuation, reduced PSTN network stability, poor telephone sidetone, increased ringer loading, and degraded termination impedance, especially with multiple telephones on the telephone line (a typical scenario). Thus, a heretofore unaddressed need exists in the industry for an improved filtering systems and methods.